Changed spell failure rate descriptions per 1714661 (slightly modified.)

The breakpoints are now at 5%, 10%, 20%, 30%, 50%, 70%, 80%, 90%, 95%.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@1485 c06c8d41-db1a-0410-9941-cceddc491573

Created by  haranp  on May 20, 2007
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